In a base station of a wireless communication network, the antenna is the port through which radio frequency (RF) energy is coupled from the transmitter to the outside world and to the receiver from the outside world. By using an array of antennas, a number of benefits are expected including improved directionality, higher signal-to-noise ratio (SNR), and better capability of interference rejection for received signals.
Manufacturers of antenna array systems formerly used signal processors that assumed ideal antenna array characteristics. Therefore, antenna arrays used in association with former signal processors had to achieve high quality standards to perform acceptably, making antenna arrays expensive. Additionally, these antenna array systems experienced degraded performance when temperatures changed, humidity levels changed, or other environmental conditions changed because the characteristics of the antenna array diverged from the expected ideal characteristics under these altered environmental conditions.
Calibration systems play an important role in modern wireless communication systems employing adaptive antenna technologies, for example antenna arrays. Depending on the applications and the signal processing algorithms employed by the base station, antenna array calibration includes determining the characteristics of the RF paths of the base station and using the characteristic data to optimize base station radio transmission and radio reception. In some cases the term “calibration” may be used to refer to the determination of system characteristics, while in other cases the term “calibration” may be used to refer to the use of the characteristic data to optimize base station radio transmission and radio reception. The meaning of the use of the term calibration will be understood by the context in which it is used. The characteristic data may be referred to as calibration data. Smart antenna systems particularly may benefit from the enhanced system performance that can be obtained from calibration.
Modern antenna array systems typically store calibration data that is then used to optimize radio transmission and reception. Conventional methods and systems for obtaining the calibration data, however, have many drawbacks. The known calibration systems may include extensive measuring equipment that is both unwieldy and expensive. Some conventional calibration methods are sensitive to drifts in system parameters, and these drifts lead to inaccuracies in the calibration data obtained using these methods. To avoid these difficulties, some antenna arrays are assigned calibration data that is generic for their particular design but that does not represent the characteristics unique to the individual antenna array. Because conventional calibration methods may be sufficiently time-consuming that periodic recalibration is impractical, some antenna arrays are calibrated only in the factory or upon initial installation, and thereafter their characteristics may diverge from the factory calibration data as the antenna array ages or as environmental conditions change. Antenna arrays that have not been individually and recently calibrated in their current environment may have inaccuracies in their array calibration data that may result in performance degradation.
Current approaches to overcome these difficulties with antenna array calibration techniques may provide a calibration system built into the base station. Calibration methods and systems may be designed that minimize the duration of time required to measure antenna array calibration data. Current calibration systems, however, are typically designed for specific vendors and systems, are manufactured in low volume, and are relatively expensive.
Therefore, there is a need in the art for an improved method for calibrating base stations. In particular, there is a need for a less expensive calibration method that is capable of calibrating base stations that operate using any technology, such as Code Division Multiple Access (CDMA) or Orthogonal Frequency Division Multiplexing (OFDM), and using any operational mode, such as frequency division duplexing (FDD) or time division duplexing (TDD).